1. Field of the Invention
The present invention relates to a two-phase stainless steel containing 10-75% ferrite with the balance being austenite and having a remarkably excellent hot workability.
2. Description of the Prior Art
In recent years, two-phase stainless steels have been increasingly used in various applications for their excellent corrosion resistance, particularly stress corrosion resistance, and their excellent resistance to welding cracks. However, simply because the two-phase stainless steels contain simultaneously the ferrite phase and the austenite phase, they are more susceptible to the interfacial cracking between the ferrite phase and the austenite phase during the hot workings, such as the break-down rolling and the hot rolling, and it has been well known as taught by A. Gueussier and R. Castro in "Metal Treatment and Drop Forging;" October (1959) 361, that their hot workability deteriorates remarkably when the ferrite is present in an amount from 10 to 75%. For this reason, two-phase stainless steels show a very low yield ratio in the break-down rolling and the hot rolling, and even if some improvement of yield ratio can be attained on the break-down forging, they have been still confronted with operational and economical disadvantages.
Therefore, improvement of the hot workability of two-phase stainless steels has long been strongly sought for in the steel making industry.
Conventionally, sulfur and phosphorus contained as impurities in stainless steels are present usually in an amount from 0.006 to 0.02%(S) and in an amount from 0.01 to 0.03%(P) without a special melting method, such as "Electro-slag Remelting Process" and "Electro-slag Refining Process" using metallic Ca-CaFe slag as disclosed in U.S. Pat. No. 3,879,192 [hereinafter called "MSR Process" (Metal Bearing Solution Process)], or a special refining method such as "AOD" (Argon-Oxygen Decarburization) process developed by Union Carbide Corp., or strict selection of the raw materials.
Sulfur and phosphorus contents as impurities in the above ranges do not substantially produce adverse effect on the hot workability in case of ferritic stainless steels, and also in case of austenitic stainless steels, their adverse effect is not so remarkable. However, in case of two-phase stainless steels, sulfur and phosphorus contents even in the above ranges deteriorate the hot workability remarkably.